Liquify

Mode

The operating mode of the modifier is selected here:

Liquify: In this mode, the specific properties of a liquid are added to standard particles. These are properties such as Viscosity, Surface tension, behaviour in Collisions and Damping. This function is only applied to standard particles. Particles that have already been given the properties of a liquid are not processed again by the modifier in this setting. This conversion of the particles can be reversed at any time by using the Unliquify Mode.
Please note that liquid particles have additional physical properties compared to standard particles and that not all particle modifiers can work as usual due to automatically acting forces on these particles.
Set Properties: This mode allows you to change the properties of liquid particles. Liquid particles are those particles that already have a Liquid Contribution property. Standard particles are not processed by the modifier in this mode and remain unchanged. These must first be converted to liquid particles using the Liquify mode of the modifier.
Unliquify: In this mode, liquid particles are converted to standard particles. The typical properties of a liquid are lost as a result. This conversion can be reversed at any time by using the Liquify Mode.

Standard particles and liquid particles differ in their behavior and the forces acting on them:

Gravity: Liquid particles are automatically influenced by gravity, which is defined in the Scene Settings, whereas standard particles require separate Force Objects to apply a gravitational force to the trajectories.
Collisions: Liquid particles automatically react to other dynamic objects, such as Rigid Bodies or Cloth, without the need for a Collide Modifier. The same applies to interaction with objects that have a Collider Tag. Standard particles must be processed by a Collide Modifier in order to react to other objects. The force that the particles exert on rigid bodies, for example, can be controlled via their mass. You will find a corresponding setting in the collision section of the particle properties on the Liquid Fill Emitter or on the Liquify modifier.
Particle-particle interaction: Liquid particles react automatically to other liquid particles in their vicinity. For example, liquid particles automatically avoid colliding with each other. At the same time, however, attractive forces also act between liquid particles, which are determined by properties such as Surface Tension. In addition, properties such as Viscosity influence the individual velocities of the liquid particles.
Physical properties: Liquid particles have an individual mass and can therefore have an effect on other dynamic objects. Liquid particles can therefore also deform cloth or move dynamic objects, for example. Standard particles can react to dynamic objects with a collision by using a Collide Modifier, but cannot themselves exert an influence on these simulation objects.

Use Default Radius

If this option is active, the particles are automatically assigned the Default Radius, which can be configured via the Liquid Settings within the Scene Settings. Please note that the Radius assigned by the emitter for the default particles may differ from this. Switch this option off if the original Radius of the particles is to be retained.

In general, it should be remembered that liquids should use a homogeneous Radius value for the particles. Varying the radii during the liquid simulation can lead to unpredictable attraction or repulsion reactions. Note this especially when subsequently converting standard particles to liquid particles, as e.g. standard emitters also offer the option of automatically varying the particle radii on emission.

Liquid Contribution[0..100%]

This value determines how strong the behavior of the particles of a liquid should be, including external forces, such as automatically acting Gravity (see Scene Settings) or automatic collision with dynamic objects, as well as internal forces of attraction and repulsion between the particles, which simulate the Surface Tension and Viscosity of the liquid, for example.

This property is only available for liquid particles and makes it possible to mix the behaviour of standard particles and liquid particles. At 0%, liquid particles then also behave like standard particles, while at 100% the complete behaviour of liquids is simulated. Remember that already emitted standard particles must first be converted to liquid particles using the Liquify Mode of the Liquify modifier in order to be able to change this property in the Set Properties Mode of the Liquify modifier during the simulation. As a rule, you will combine the Liquefy Mode of the modifier with a Liquid Contribution of 100% in order to carry out a complete conversion from standard particles to liquid particles.
When converting liquid particles to standard particles by using Unliquify Mode, this Liquid Contribution property is automatically removed from the particles.

Constraints

In this group you will find the settings that describe the behavior and properties of the liquid particles. These properties are transferred to the particles in the Liquify and Set Properties Modes.

Target Density[0.00..+∞]

This can be used to control the mass density and thus, for example, the buoyancy of the liquid. This influences the interaction with other liquids, but also, for example, with solids or with objects defined as cloth. In this context, please also note the setting for the Interaction Mass in the Collisions settings, as this also influences the interaction between liquids and other dynamic objects.

Here, three lifebuoys were configured with different densities using their Rigid Body tags. Accordingly, objects floating on the liquid, moving with the liquid or even sinking can be realized.

When liquids with different densities meet, the liquid with the higher density will sink. Think of it like the meeting of water and oil. The oil will separate from the water and float on the water. If you want even greater separation of different liquids so that no mixture is calculated between them, you can also use different values for the Mixture ID. Only liquid particles that have an identical Mixture ID value can then interact with each other, e.g. in terms of Surface Tension.
The density also has an effect on the distribution behavior of the liquids. If a low density is specified, the individual particles repel each other more strongly and the liquid takes up more space overall. With higher density values, more liquid particles can be held together in a small space without causing repulsion effects.

The following video gives an example. There, three Basic Emitters generate liquid particles and manage them in three separate particle groups. The red particles are given a Target Density of 50, the orange particles a Target Density of 500 and the yellow particles a Target Density of 5000. It is good to observe how the red particles at the bottom spread out quickly in order to keep the density there as low as possible. This is in stark contrast to the yellow particles, which have no problem concentrating even in a small area due to the high density requirement.

Increasing values for the Target Density of the liquid particles can be seen here from left to right.

Viscosity[0.00..+∞]

This value describes the resistance of a liquid to flow. Higher values lead to viscous liquids such as toothpaste, caramel or honey (e.g. 0.8), lower values stand for milk, oil or water (e.g. 0.05). This effect can be further enhanced by increasing the Target Density and Damping values.

Examples of low (left) and high Viscosity values (right).

Surface Tension[0.00..+∞]

This defines the force of attraction between neighboring liquid particles. Higher values lead to more attraction and a sticking together of neighboring particles, as can be observed, for example, with water droplets touching each other. At high values, this can also lead to the particles forming separate groups more quickly, which then form thread-like or drop-shaped structures. It should be noted that the Target Density can also have an influence on this effect. If high Surface Tensions are used, this leads to clumping of particles. If this causes the number of particles per volume to rise above the Target Density, a counter-reaction can occur, which then pushes the particles apart again. It is then necessary to find a suitable relationship between these values.

Here you can see examples of different values for the Surface Tension. On the left a low value of 0.05 and on the right a value of 10.

Mixture ID[-2147483648..2147483647]

This value can be used to separate liquids from each other, although other properties, such as the Target Density, are otherwise identical. Only the liquid particles that have the same Mixture ID value move homogeneously and can mix with all other particles as desired. Particles with different Mixture IDs automatically separate from each other and only react with particles of their own Mixture ID group, e.g. with regard to Surface Tension. The effect is similar to the use of different Target Densities, but does not involve different buoyancy behavior.

Technically speaking, liquids with identical Viscosity and Surface Tension but different Mixture IDs behave as if there is no friction between them. If, in addition to different Mixture IDs, different Viscosities or Surface Tensions are also used, the liquids also tend to repel each other.

On the left, both particle streams have the same Mixture ID value; on the right, different values were used.

Ease In

This value specified in animation frames indicates the period of time during which the particles should obtain the new liquid properties. This is mainly helpful for the generation of liquid particles at standard emitters(Basic Emitter, Spline Emitter, Mesh Emitter), as new particles are created at random distances from each other. This often results in overlapping liquid particles, which would then repel each other violently. The same problem can occur if existing standard particles are converted to liquid particles using the Liquify Modifier. In these cases, too, the initial particles may already be too close together, which then leads to an immediate rejection reaction on the liquid particles.

This problem is alleviated by allowing the particles a certain amount of time before the full properties of the liquid are included in the simulation. The particles can slowly balance their attractive and repulsive forces without causing explosive reactions on the liquid particles.

By the way:

Since the particles at the Liquid Fill Emitter are already formed in a fixed grid, which prevents initial collisions and overlapping, no transition time is normally necessary there.

Here you can see Basic Emitters that generate liquid particles without their own velocity. On the left no Ease In was used, on the right a Ease In of 5 frames. The new particles therefore have more time to correct their distances to their neighbors in order to avoid overlaps, which lead to the random spread of the particles at the left emitter.

Collisions

Use these values to specify how the liquid particles should behave when they collide with other dynamic objects in the simulation scene, such as objects simulated as Rigid Bodies or Cloth, as well as objects that have a Collider Tag.

Interaction Mass[0.00..+∞]

This is used to specify the mass of each liquid particle. This mass plays a role when liquid particles collide with dynamic simulation objects, such as rigid bodies or clothing, because together with the speed of the particles, it defines the force that particles can exert. The greater the mass of the colliding particles, the more they can press against a fabric or rigid body object, for example, and possibly deform or displace it.
Also consider the connection with the Target Density of the liquid particles. With a high particle density, an overall large force effect on other dynamic simulation objects can be developed even with a small mass per particle.

These simulations differ only in the assigned mass of the liquid particles. A small mass was used on the left. The rocker tilts only slightly when the particles hit it and then swings back to its original position. On the right, on the other hand, the higher mass of the particles ensures a clear reaction of the seesaw, which consists of two rigid body objects.

As can be seen in the example below, a similar effect can also be used when defining dynamic objects using Rigid Body Tags. The three lifebuoys were assigned different Mass values. Alternatively, it is also possible to assign any Density, although the dimensions of the dynamic objects then also play a role in the Mass calculation.

The blue ring was given a Mass of 1.2, the green ring a Mass of 0.75 and the red ring a Mass of 0.3. The simulated water has a Mass of 1 with a Density of 1000. The ring with the greater mass sinks in the water as a result. Depending on the flow, the ring with the medium mass can float on the liquid or float within the liquid, whereas the lightest ring floats permanently on the water.

Here, three ring objects were assigned different Mass values via Rigid Body tags. This results in the different behavior of the otherwise identical objects. A ring sinks directly due to a large mass allocation, a ring with a mass comparable to the liquid moves with the liquid and a ring with a low mass floats permanently on the surface.

Friction[0.00..+∞]

This value describes how much kinetic energy the liquid should lose when it comes into contact with a dynamically simulated object or an object marked with a Collider tag. A liquid spilled on a floor can thus be automatically slowed down. It should be noted that this friction is also always dependent on the Friction value, which was then specified on the Collider tag of an object, for example. Both Friction values of the colliding simulation elements interact with each other. If, for example, a plane used as a floor has a Friction of 0, a liquid with a high Friction value will also flow along the plane unchecked. They therefore only ever indicate the property of the colliding object or the respective liquids with the respective Friction value.

Both simulations are identical except for the Friction value. Only on the right has the liquid been assigned a Friction above 0. In both cases, the floor level was given a Friction of 0.2 via a Collider tag.

Stickiness[0.00..+∞]

This effect is even stronger than the Friction described above, because the Stickiness activates a force of attraction when the liquid comes into contact with the surfaces of dynamic objects. In this way, a liquid can also adhere to vertical surfaces, for example, or only drip off there at a slower rate.
As already explained for the Friction property, there are always two components to Stickiness: the Stickiness of the liquid particles and the Stickiness of the surface that collides with them. If one of these components is not Sticky, the liquid cannot adhere to it.

Both simulations are identical except for the Stickiness value. Only on the right has the liquid been assigned a Stickiness above 0. In both cases, the geometry of the steps was given a Stickiness of 0.1 via a Collider tag.

Damping

Damping[0..+∞%]

This value specifies how much kinetic energy should be removed from the simulation of the fluid particles frame by frame. High values can quickly bring a simulation to a complete standstill. Even small values can help to mitigate overshooting movements and explosive accelerations within the simulation. For this reason, the default value of 2% for Damping in simulations is already adopted by the emitters. This default value can be found in the Scene Settings.